Backend interview prep that plans your day, checks every answer and remembers what you miss.

Pick 30, 60 or 90 days. Daily work from your weakest areas across DSA, system design, Java, SQL and CS core.Pick 30, 60 or 90 days. 90x picks each day's work from your weakest areas across DSA, system design, Java, SQL and CS core, and checks every answer.

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Answer a card.
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It comes back before you forget.

Back tomorrow if missed, in a month if right.

System design · Medium · Pick oneYour API retries failed calls instantly. When the database slows down, traffic triples and it falls over. What should the retries use?

A longer timeoutBackoff with jitterMore replicasA bigger pool

Correct100%

Today+1+30

problems: 3,693
lessons: 273
sources: 44
on GitHub, MIT: Code

A feed that grades you back.

Ten kinds of card. Every one marked.

SQLPick one

Which ACID property means all of a transaction's writes apply, or none do?

ADurability

BAtomicity

CIsolation

DConsistency

Correct

BehaviouralWrite it out

Tell me about a time you received difficult feedback.

Names a specific piece of feedback

States what you changed

Says what happened afterwards

Not quite · 1 point missed

DSATick the grid

Tick the order each structure removes items in.

StackLIFO ✓FIFO

QueueLIFOFIFO ✓

Correct

CS coreSort

Sort each use by the transport it runs over.

UDPDNS query · Live video call

TCPHTTP/1.1 · SSH session

Correct

SQLPick one

Which ACID property means all of a transaction's writes apply, or none do?

ADurability

BAtomicity

CIsolation

DConsistency

Correct

BehaviouralWrite it out

Tell me about a time you received difficult feedback.

Names a specific piece of feedback

States what you changed

Says what happened afterwards

Not quite · 1 point missed

DSATick the grid

Tick the order each structure removes items in.

StackLIFO ✓FIFO

QueueLIFOFIFO ✓

Correct

CS coreSort

Sort each use by the transport it runs over.

UDPDNS query · Live video call

TCPHTTP/1.1 · SSH session

Correct

DSATap the line

Tap the line that breaks find_max for some inputs.

1def find_max(nums):

2 best = 0

3 for n in nums:

4 if n > best:

5 best = n

6 return best

Not quite · back tomorrow

CS coreOrder

Put the steps of a TCP connection in order.

1SYN

2SYN-ACK

3ACK

4FIN

5Data transfer

Not quite · back tomorrow

System designTrue or false

N = 3. Mark each statement.

TW=2, R=2 always reads the latest acknowledged write

TW=1, R=1 can return a stale value

FQuorums alone order concurrent writes

Not quite

DSATap the line

Tap the line that breaks find_max for some inputs.

1def find_max(nums):

2 best = 0

3 for n in nums:

4 if n > best:

5 best = n

6 return best

Not quite · back tomorrow

CS coreOrder

Put the steps of a TCP connection in order.

1SYN

2SYN-ACK

3ACK

4FIN

5Data transfer

Not quite · back tomorrow

System designTrue or false

N = 3. Mark each statement.

TW=2, R=2 always reads the latest acknowledged write

TW=1, R=1 can return a stale value

FQuorums alone order concurrent writes

Not quite

CS coreNumber

How many bits give 200 values their own pattern?8Correct

CS coreMatch

Pair each HTTP status code with its meaning.

301Moved permanently

401Not authenticated

404Not found

503Service unavailable

Correct

SQLAssemble

Build the query: customers with more than 5 orders.

SELECT customer_id, COUNT(*)

FROM orders

GROUP BY customer_id

HAVING COUNT(*) > 5

Correct

Ten kinds of card. Every one marked.

SQLPick one

Which ACID property means all of a transaction's writes apply, or none do?

ADurability

BAtomicity

CIsolation

DConsistency

Correct

DSATap the line

Tap the line that breaks find_max for some inputs.

1def find_max(nums):

2 best = 0

3 for n in nums:

4 if n > best:

5 best = n

6 return best

Not quite · back tomorrow

CS coreNumber

How many bits give 200 values their own pattern?8Correct

BehaviouralWrite it out

Tell me about a time you received difficult feedback.

Names a specific piece of feedback

States what you changed

Says what happened afterwards

Not quite · 1 point missed

CS coreOrder

Put the steps of a TCP connection in order.

1SYN

2SYN-ACK

3ACK

4FIN

5Data transfer

Not quite · back tomorrow

CS coreMatch

Pair each HTTP status code with its meaning.

301Moved permanently

401Not authenticated

404Not found

503Service unavailable

Correct

DSATick the grid

Tick the order each structure removes items in.

StackLIFO ✓FIFO

QueueLIFOFIFO ✓

Correct

System designTrue or false

N = 3. Mark each statement.

TW=2, R=2 always reads the latest acknowledged write

TW=1, R=1 can return a stale value

FQuorums alone order concurrent writes

Not quite

SQLAssemble

Build the query: customers with more than 5 orders.

SELECT customer_id, COUNT(*)

FROM orders

GROUP BY customer_id

HAVING COUNT(*) > 5

Correct

CS coreSort

Sort each use by the transport it runs over.

UDPDNS query · Live video call

TCPHTTP/1.1 · SSH session

Correct

SQLPick one

Which ACID property means all of a transaction's writes apply, or none do?

ADurability

BAtomicity

CIsolation

DConsistency

Correct

DSATap the line

Tap the line that breaks find_max for some inputs.

1def find_max(nums):

2 best = 0

3 for n in nums:

4 if n > best:

5 best = n

6 return best

Not quite · back tomorrow

CS coreNumber

How many bits give 200 values their own pattern?8Correct

BehaviouralWrite it out

Tell me about a time you received difficult feedback.

Names a specific piece of feedback

States what you changed

Says what happened afterwards

Not quite · 1 point missed

CS coreOrder

Put the steps of a TCP connection in order.

1SYN

2SYN-ACK

3ACK

4FIN

5Data transfer

Not quite · back tomorrow

CS coreMatch

Pair each HTTP status code with its meaning.

301Moved permanently

401Not authenticated

404Not found

503Service unavailable

Correct

DSATick the grid

Tick the order each structure removes items in.

StackLIFO ✓FIFO

QueueLIFOFIFO ✓

Correct

System designTrue or false

N = 3. Mark each statement.

TW=2, R=2 always reads the latest acknowledged write

TW=1, R=1 can return a stale value

FQuorums alone order concurrent writes

Not quite

SQLAssemble

Build the query: customers with more than 5 orders.

SELECT customer_id, COUNT(*)

FROM orders

GROUP BY customer_id

HAVING COUNT(*) > 5

Correct

CS coreSort

Sort each use by the transport it runs over.

UDPDNS query · Live video call

TCPHTTP/1.1 · SSH session

Correct

How a day works.

Your day's work, planned for you.

Today picks your missions.
Each morning Today builds your session from your weakest areas and the reviews that are due.
You answer and get graded.
Every card is checked the moment you answer, with the right answer and the reason behind it.
Readiness moves.
Readiness rises only for the work you did, and what you missed comes back before you forget it.

Day 1 starts with a card.

Backend interview prep. Five areas, one plan.

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Backend interview prep that plans your day, checks every answer and remembers what you miss.Backend interview prepthat plans your day.that checks every answer.that remembers what you miss.that plans your day.

Watch Ren mark a card.

A feed that grades you back.A feed that grades you back.

Ten kinds of card. Every one marked.

How a day works.

Today picks your missions.

You answer and get graded.

Readiness moves.

How a day works.

Today picks your missions.

You answer and get graded.

Readiness moves.

Day 1 starts with a card.

Backend interview prep that plans your day, checks every answer and remembers what you miss.

A feed that grades you back.